YEAH, REPLACDE IT WITH A 5.9 CUMMINS LIKE I DID.

 

typical non 6.5 owner

 

First, let's find out what your plan is for this truck.

Sled puller, daily driver, towing rig, roller queen, etc.

Once you figure out what you want to do with the truck, then you can get some relevant answers.

The GMx series turbo's are built by Borg Warner. They are very good high quality turbos and work very well in a stock application where long life, mpg and daily driving is the target.

However, they are built for instant boost response for day to day driving. GM wanted this to sell trucks. IE: when the average consumer stomps on the pedal and they fell abrupt acceleration they think the truck is powerful. In actual fact, the truck has been "tuned" as a system to give that impression and it's "all in" very low in the rpm range. It pulls like a mule to 2200, then it falls flat. You've got lots of pedal travel left to the floor, but it's all pretty useless. The trade off to get this is the turbine is designed to extract as much energy out of the exhaust flow as soon as possible.

Not a big deal if you are just driving around town and operate around the torque peak at light/medium loads.

My truck with a fresh GM8 will rip both rear tires for 15-20 feet if I just hammer the throttle from a stop. It also pulls my 8600 lb trailer fairly easily. But I know it's limits and never exceed them.

The problem with the GMx turbos is when you want to run the engine higher up in the RPM range and try to extract more boost out of them, the turbine starts becoming as much a restriction in the exhaust as anything else and the compressor rapidly gets out of it's efficiency range on the map. The drive pressure goes up and the compressor efficiency comes down.

Result?

High EGT's at relatively low power levels and/or high towing loads.

They are also designed to provide low levels of boost since the 6.5 is a relatively high compression diesel (can be as high as 22:1, depending on the year). If you look at the "archaic" turbo map for an IHI RCH6 (What GMx turbos are) it maxes out the compressor at 25 psi. The compressor exducer is entering sonic speeds at the tips at this point and compressors cannot deal with sonic air. This is what GM spec'd them for, this is what IHI/Borg Warner built. You don't want to spin them this high anyways; the speeds are beyond spec (somewhere around 130,000 rpm) and the heat in the compressed air is excessive. Drive pressures are also through the roof. Most GMx turbos are going to max out closer to 20 PSI anyways. Still waaaay too high for the design...

There is also no aftermarket support for them so you can forget different turbine and compressor wheels.

The turbo referenced above is the ATT. It was "developed" by a gent on another forum. It's a slightly modified Mitsubishi tdo7 non wastegated unit. It's used primarily on Hino trucks IIRC.

The ATT is built to lower EGT's while towing. It doesn't have the response down low like the GMx series turbo's, but it's not objectionable either from what I understand from a couple guys that use them. Where it really is supposed to outdo the GMx is after 2200-2400 rpm. Where the GMx is getting out of it's efficiency range by then, the ATT is still in the good part of the map due to lower drive pressures required.

Now, that's mostly for towing.

If you want performance, that's a whole different ball of wax......